This is a Phase III multicenter, randomized, double-blind, placebo-controlled study designed to assess the efficacy of CRL 5861 (Flocor) in reducing the duration of vaso-occlusive crisis in patients with sickle cell disease. Patients will be randomized to one of two arms (placebo or Flocor) and will receive a loading dose followed by a maintenance infusion of the study drug within 12 hours from presentation to the hospital. Crisis evaluations will be made during treatment, post-treatment, and follow-up. The sickle cell diseases are a group of hereditary hemoglobin disorders characterized by presence of abnormal red cells, which undergo sickle shape transformation when deoxygenated. The most common manifestations of sickle cell disease (SCD) are hemolytic anemia and vaso-occlusive complications. It is generally accepted that the acute painful episode or vaso-occlusive crisis of SCD results from microvascular occlusion. Although the mechanisms leading to vaso-occlusion are incompletely understood, the impaired deformability and increased adhesiveness of sickled red blood cells are clearly important. As the number of less deformable cells and irreversibly sickled cells increases, the viscosity of whole blood increases. Current therapy, with the exception of bone marrow transplantation, is usually limited to the prevention of infections and management of complications. The active component of Flocor is purified polozamer 188, a nonionic, block copolymer surfacant with hemorrheologic and antithrombotic properties. It improves microvascular blood flow by reducing viscosity and by reducing adhesive frictional forces. The primary aim of the study is to access the efficacy of Flocor compared to placebo in reducing the duration of vaso-occlusive crisis in patients with SCD. The secondary objective is to access the effects of Flocor on the duration and intensity of pain, the total analgesic use and length of hospitalization of patients with SCD who are experiencing a crisis; to obtain pharmacokinetic data in these patients; and to compare pharmacoeconmic difference between patients administered Flocor and placebo.